The invention relates to a beam warping machine with a device for the maintenance of required yarn tension in a group of yarns with rolls around which said group of yarns is wound following an S-shaped path in continuous operation, said pair of rolls being driven without slippage in a operative position whereby brakes acting upon the warp beam are activated when the machine is stopped while at the same time one of the rolls is swivelled radially out of its operative position and away from the yarn group.
Such a machine is known from German document DE-OS 31 49 082.
Such a machine must be taken out of operation by stopping it when a yarn breakage occurs for example, or when a thread end (breakage of yarn filament) is detected. To detect such defects, known monitoring instruments are provided. When a defect is detected the machine involved must be stopped as quickly as possible so that the defective area may not be wound on the warp beam. For this reason, the stopping process must be especially short and take place within less than one second, for example.
The defects described are such as normally affect only one yarn out of a multitude of yarns, e.g. 1000 yarns, so that it is necessary to repair the defect concerned individually, with the winding process continued thereafter. However, the yarn material must not suffer any damage during a stoppage. It is even especially desirable for no change in the buildup of the yarns on the warp beam to occur in the area of the defect, due to the stopping procedure, in addition to the original defect which is repaired by a knot, for example. Such a single knot generally does not attract attention in the fabric produced later on. Considerable change in the buildup of the yarns however, such as for example brief, considerably greater or lower yarn tension due to the stoppage would appear later on in the form of stripes in the finished fabric, detracting from the general appearance of the goods. To be especially avoided is the production of such a great increase in yarn tension, due to rapid stoppage, that damages occur in a number of yarns, as this would in a way multiply the defect originally limited to one single yarn. Slacking of the yarns must also be avoided, however, as it may easily lead to yarn ravelling.
According to DE-OS 31 49 082, the above noted problem is dealt with by swivelling one of the rolls out of the group of yarns to such an extent, when the device is stopped, that the yarns run past this roll at a distance, thus practically preventing that moved away roll from exerting any friction upon the yarns because these are no longer wound around the rolls. To maintain a minimum yarn tension during this process, care must be taken that the warp beam continues to run at first during swivelling of the roll, coming to a stop only when no further yarn length is freed by the swivelling motion of said roll, as is the case when the roll no longer touches the yarns. In order to ensure continued minimum yarn tension when the warp beam is stopped and the roll is swivelled into the outer position, an individual stop brake, activated together with the swivelling of the roll, is provided for each yarn. Aside from the considerable cost involved with individual brakes because of the great number of yarns, it is difficult to synchronize the different events, i.e. the braking of the warp beam, the swivelling of the roll and the activation of the stop brakes so that a minimum yarn tension is also maintained with the desired uniformity.